Prior art valves suitable for this purpose consist of two cylindrical parts, each of which is provided with. two butterflies on its inside that in normal conditions are closed since they are kept in position by appropriate means of engagement and, when they become detached from these means, open under the action of springs to interrupt the oil flow in the duct. It should be clarified that the term "butterflies closed" is hereinafter to be understood as referring to the position in which they are held when the duct is open and operating, while "butterflies open" refers to the position they assume after being released to close the duct. Both the cylindrical parts are provided with circular flanges along the entire outer perimeter of the cylinder, so that they can be fixed to each other by means of appropriate bolts. In normal operating conditions these bolts maintain the valve assembled and the separation of the two parts of the valve following a breakaway occurs as a result of the failure of the said bolts, which are designed and calibrated to fail at a given traction force. Indeed, the separation of the two parts of the valve must necessarily occur before the duct actually breaks. The best known valves of this prior art type are manufactured by the Atema, Gail Thomson and MIB companies.
Document US A-5 564 471 is known that discloses a fuel hose breakaway unit in which fixed abutment lugs are formed on one of the couplers and are shaped to engage force fingers carried by the other coupler; moveable lugs having ramped abutment surfaces are located on the fingers and engage ramped surfaces on the fixed abutment lugs to lock the couplers together; a sleeve is adjustably mounted on the force fingers to maintain the engagement of the couplers. In the assembled position, the sleeve prevents separation of the force fingers and fixed lugs. A spring unit is disposed adjacent to the sleeve and is calibrated so that when a separation force is applied to the unit, the spring compresses and if the separation force exceeds a predetermined value the spring pushes the sleeve away from the force fingers, allowing the fingers to flex outwardly away from the lugs to decouple the unit. The spring unit also absorbs line shock forces applied to the breakaway unit in order to limit the effects of these forces on the remaining components in the unit.
The principal drawback of the prior art valves referred to in the first list here above is that the fixing system of the two parts of the valve requires all the fixing elements to be substituted following a breakage of the duct, while a second drawback is constituted by the fact that the calibrated bolts may not all fail at the same time when the valve is subjected to bending in addition to tension; in that case the two parts of the valve would separate on one side and remain in contact on the other. Such a partial misaligment would not cause the butterflies to become released, so that oil would gush from the duct. A further drawback deriving from this fixing system is the considerable weight and size of valves of this type.
The drawbacks are considered with reference to the fact that the valve disclosed in US A-5 564 471 is adapted for mounting in a fuel hose between a fuel dispenser and nozel; on the contrary, the valve of the present invention is particularly adapted for use in submarine conditions wherein ducts have dimensions that are multiple of those in a fuel hose. The valve disclosed in US A-5 564 471 presents a mechanical complication involving a number of finely worked pieces; the use of a spring unit which should be sturdy enough to face its duty and would result of heavy weight and cost. As a whole, this valve does not appear adapted to work in said submarine conditions which would likely entail malfunctioning of the parts due to wearing and corrosion.